1. Field of the Invention
This invention relates to the elimination of polluting constituents from gaseous streams. The invention is particularly applicable to the removal of solid particulates from a high temperature gas.
2. Prior Art
The problems of air pollution as a result of the venting of waste gases have been long recognized and the subject of a substantial amount of current research. Various techniques are being used to treat waste gases to separate, for example, fly ash from flue gases. In such treatment, gas filters, dynamic air-solids separators, wet scrubbers and electrostatic precipitators are used. All of these techniques, however, have certain disadvantages and very few of them are effective in the removal of gas phase impurities from such gaseous streams.
Bag filters or fiber fabric filters and rigid porous filters, for example, are fairly effective in removing solid contaminants from gaseous streams. However, such filters soon become clogged with the gas entrained particles which are collected, resulting in an unacceptable increase in pressure drop across the filter and necessitating the use of some means to clean the surface of the filters. One means, for example, to provide continuous filtering capability is to have a dual system, such that the gas can be diverted to one filter while the other is being back-flushed or cleaned. Another means is to provide a short pulse of pressure in a direction reverse to the normal flow. Still another means is to rap or shake the filter. When ceramic fibers are used to form a filter suitable for high temperature operation, the back flow pressure pulse and/or vibration used to clean them results in a breakage of the ceramic fibers and a substantially shorter life for the filter.
Another technique utilized to remove gas entrained particulates is to pass a contaminated gas through a bed of granular material such as sand. The gas-entrained particulates are collected on the upstream surface of and within the sand bed. The fixed bed suffers from the same disadvantage as the fiber or porous type filters discussed before, i.e., gas flow must be stopped for cleaning. The fluidized bed is continuous in operation but is less effective in removing small particles than a fixed or dense packed moving bed. Furthermore, it requires a substantially uniform flow of gas. More particularly, any sudden surge of gas going through the fluidized bed can result in not only the previously contained particulates passing through the bed but also a portion of the bed itself being entrained in the gas, thus adding even more to the particulate loading of the gas stream.
Moving beds are more efficient filters than fluidized beds (for equal bed thickness and material) but are not as efficient as static beds. It is believed that continuous motion of the bed dislodges some of the fine, collected particles which can then gradually migrate through the bed due to the force of the flowing gas. Nevertheless, this approach is useful and is the subject of several patents. For example, U.S. Pat. No. 1,570,869 discloses a two-stage arrangement in which the gas passes through a moving bed of granular material at least twice in order to enhance the removal of small particulates. A more recent patent, U.S. Pat. No. 4,017,278 describes the use of an annular moving bed flowing between concentric vertical cylinder walls which are perforated to allow the gas to pass through the bed into the space enclosed by the interior cylindrical wall. Moving beds have also been proposed for the removal of gaseous impurities.
U.S. Pat. No. 3,589,863 discloses a method of removing sulfur dioxide and particulate matter from gas streams. The acid gases and particulate matter are removed from gaseous streams by a process which comprises passing the gaseous stream through a bed of porous alkali metal bicarbonate aggregrates. It is disclosed that the bed can be a fixed system or a counter-currently moving replenished system. The porous bicarbonate aggregates attract the particulate matter to their surfaces and thus remove them from the stream, while the acid gas is simultaneously reacted within or around the aggregates to form solids which leave the gaseous stream.
Another example of a moving bed is shown in U.S. Pat. No. 3,716,969 which discloses the use of two baffle walls which are downwardly inclined in a manner to provide an increase in the space between them in the downward direction. This is supposed to facilitate the downward flow of a body of granular material between the walls.
U.S. Pat. No. 3,296,775 discloses a method and apparatus for treating fluids and non-fluid materials which utilizes a moving bed of granular material. The patent is particularly concerned with the periodic removal of the granular material adjacent to the fluid entry face of the granular material bed (i.e., the surface) while retaining the remainder of the bed. The patentee suggests periodically causing a surge flow of a gas which sharply peaks substantially above and substantially immediately declines below the minimum steady flow rate at which the flow of gas causes motion of the granular material and in a reverse direction through the fluid entry portions of the apparatus to remove a portion of the granular material from the bed. Thus, particulates entrained in the gas which are collected on that surface are removed along with a portion of the granular material. The principal disadvantage of granular bed filters is that either a very thick bed or very fine granular material (or both) are required to give high efficiency removal of particulates in the 0.5 to 10-micron size range. The requirement for thick beds results in large expensive equipment, while the use of very fine granular material causes high pressure drop, poor bed flow characteristics, and bed particle entrainment in the product gas.
Wet scrubbers also are used for the removal of particulates and overcome some of the disadvantages hereinbefore described, namely, that of requiring periodic shutdown and cleaning. However, wet scrubbers also saturate the gas stream with moisture. Thus, the wet scrubbers cannot be used on hot, dry gas streams without cooling them to approximately the temperature representing equilibrium with liquid water at the scrubber exit conditions. This is an obvious waste of thermal energy and undesirable if a clean high temperature gas is required. Furthermore, when the gas is discharged to the atmosphere, upon further cooling, the moisture condenses causing an objectionable plume. Dynamic mechanical devices also have been used, a typical example being a cyclone scrubber. These devices also do not require frequent shutdown for cleaning. However, they are relatively ineffective for particles less than about 5 microns in diameter.
Electrostatic precipitators are frequently used for the removal of particulates from gaseous streams. Such devices have been found to be fairly efficient, however, a disadvantage of the electrostatic precipitator is that efficiency is affected by the electric resistivity of the particles, chemical composition of the gas, temperature and other factors. As a result, they are not economically applicable to many applications.
Thus, it is seen that there is still substantial room for improvement in the removal of particulate contaminants from gaseous streams. More particularly, the ideal particulate contaminant removal technique should be relatively simple, have a high efficiency for a broad range of contaminant sizes, should also be amenable to the removal of gas phase impurities, should not require shutdown for cleaning and should have a low and substantially constant pressure drop during operation. In addition, the technique should be applicable to both high and low temperature gas streams.